In electrophotographic image formation, an electrostatic latent image is formed on a photoconductive image bearer, a charged toner is attached to the electrostatic latent image to form a visual toner image, the toner image is transferred onto a recording medium such as a paper and fixed thereon. Recently, electrophotographic copiers and printers have rapidly developed from monochrome to full-color, and the full-color market is expanding.
Electrophotographic full-color image formation typically uses three primary colors yellow, magenta and cyan toners or four color toners including a black toner, and overlaps each of the color toner images to reproduce all colors. Conventionally, a one-component developing method, a two-component developing method and a hybrid developing methods are used. In order to produce uniform and clear full-color images having good color-reproducibility, a toner amount on an electrostatic latent image bearer needs to be faithfully maintained according to an electrostatic latent image. When the toner amount on the electrostatic latent image bearer varies, the resultant image varies in image density and color tone on a recording medium.
The toner amount on the electrostatic latent image bearer varies because the toner varies in charge quantity and Japanese Patent No. 4337523 (Japanese published unexamined application No. 2005-157002-A) discloses a following image takes over a history of the last image (ghost phenomenon) in the hybrid developing methods. The ghost phenomenon disclosed in Japanese Patent No. 4337523 (Japanese published unexamined application No. 2005-157002-A) is a specific problem of the hybrid developing method. The toner amount on a toner bearer varies according to a toner consumption pattern of the last image and the following image varies in image density. This is because, in the hybrid developing method, a specific amount of a toner is constantly fed to the toner bearer and the amount of a toner thereon varies according to the number of receiving a toner. Namely, after an image consuming less toner is printed, the toner remaining on the toner bearer increases, and after the toner is fed, the toner amount on the toner bearer further increases, and the resultant image has higher image density. Meanwhile, after an image consuming much toner is printed, the toner remaining on the toner bearer decreases, and after the toner is fed, the toner amount on the toner bearer decreases, and the resultant image has lower image density.
As mentioned above, the ghost phenomenon in the hybrid developing method is caused by the toner amount variation on the toner bearer when a following image is produced according to the history of the last image because it is difficult to uniform the amount of the decreased toner after used for development and the amount of the undeveloped toner remaining on the toner bearer when the toner is transferred onto the toner bearer from a two-component developer.
In order to solve these problems, Japanese Patent No. 3356948 (Japanese published unexamined application No. 9-251237-A), and Japanese published unexamined applications Nos. 2005-157002-A and 11-231652-A disclose scraping off the toner remaining on the toner bearer therefrom with a scraper or a toner collection roller after developed and before fed again. Japanese published unexamined application No. 7-72733-A discloses a method of collecting the toner remaining on the toner bearer on a magnetic roller by potential difference between copying or papers to stabilize the toner amount on the toner bearer. Further, in order to solve the problem of history development using the magnetic brush, Japanese published unexamined application No. 7-128983-A discloses widening a half width area of a magnetic flux density of the magnetic roll to collect and feed the toner on the toner bearer. Japanese published unexamined application No. 6-92813-A discloses a method of using a non-spherical carrier to increase the surface area thereof and increasing a ratio of the carriers contacting each other to charge the carrier even at the end of the magnetic brush, narrowing a substantial gap between the developer bearer and the toner bearer to increase the toner amount fed to the toner bearer at a time, and feeding the toner until the toner bearer is saturated with the toner to maintain a specific amount of the toner on the toner bearer and prevent an influence of the last image history.
Even the two-component developing method has the ghost phenomenon. Poor separation of the developer is thought to cause the ghost phenomenon. The two-component developing method has an odd number of magnets in the developer bearer and a pair of magnets having the same polarity below the rotational axis of the developing sleeve to form a separation area where a magnetic force is almost zero. The developer naturally falls there by gravity to separate from the developer bearer. However, the carrier has a counter charge when the toner is consumed in the last image, and an image force generates between the carrier and the developer bearer and the developer does not separate at the separation area. The toner is consumed and the developer having a lowered toner concentration is fed to the developing area again, resulting in production of images having low image density. Namely, images having normal image density are produced for one cycle of the sleeve, but the image density lowers since the second cycle, resulting in the ghost phenomenon. In order to solve these problems, Japanese published unexamined application No. 11-65247-A discloses a configuration of locating a scoop roll having a magnet inside at the separation area above the developer bearer to separate the developer after developed by the magnetic force. The separated developer is further scooped up by another scoop roll, and fed to a developer stirring chamber where the toner concentration is adjusted again and the toner is charged.
Japanese published unexamined application No. 2009-230090-A discloses a long-life two-component developer including a magnetic carrier formed of a core material including a binder resin and a particulate magnetic metal oxide and a coated layer including an ionic liquid, an inorganic particulate material and a binder resin, covering the core material. The developer prevents carrier adherence, has high durability, produces high-quality full-color images, and has no image deterioration such as color contamination even when producing a number of images. Japanese published unexamined application No. 2003-43756-A discloses specifying a mixing ratio of a toner and a carrier formed of a resin in which a magnetic particles are dispersed, a fluidity after magnetized (A) and a fluidity (B) before magnetized or after demagnetized to provide a two-component developer or a supplemental developer having high fluidity, no image deterioration even when producing a number of images and good durability. Japanese published unexamined application No. 4-3868-A discloses a two-component developer using a carrier formed of a magnetic material having a hexagonal magnetoplumbite structure for high-speed development. Japanese published unexamined application No. 2008-175883-A discloses specifying the total amount of excessive Fe2O3, excessive Li2O and MgO; a content of atoms besides Li, Mg, Fe and O; and a content of Mn to form a complex ferrite including Li and Mg. The resultant carrier has a light specific gravity, a high resistivity and less variation of properties such as resistivity, magnetization and surfaceness, and a developer using the carrier has durability, reliability and produces less defective images.
Because of these reasons, a need exist for a carrier having good durability, consuming a stable amount of a toner for development without influence of the toner consumption history of the last image, producing uniform images having good color reproducibility for long periods, and preventing background fouling due to toner scattering and carrier adherence.